Active matrix type display devices show images in response to selection in row units of pixel electrodes aligned in a matrix state and to writing of voltages corresponding to the display data of the selected pixel electrodes. In order to select the pixel electrodes in row units, switching elements such as thin film transistors (TFTs) are required to be provided in the display device. In the case of using TFTs, it is necessary to provide driving circuits such as a gate driver and a source driver. The TFT is a field effect transistor comprising a semiconductor layer and three terminals, that is, a gate electrode, a source electrode, and a drain electrode. The gate electrode is connected to a gate driver and the source electrode is connected to a source driver. The source electrode is also connected to a drain electrode through the semiconductor layer.
Driving circuits are commonly integrated into an IC (integral circuit) chip, and are attached outside a panel. Even in the driving circuits attached outside the panel, TFTs are suitably used and enable high-speed operation of elements such as a gate driver and a source driver. Examples of the material widely used for a semiconductor layer of a TFT include silicon-based materials such as noncrystalline silicon (amorphous silicon), microcrystalline silicon, multicrystalline silicon (polysilicon), and monocrystalline silicon. TFTs are generally classified into a bottom gate type, in which the gate electrode is formed on a lower layer than the source electrode and the drain electrode, and a top gate type, in which the gate electrode is formed on an upper layer than the source electrode and the drain electrode.
The material to be used for a semiconductor layer and selection of the bottom gate type or the top gate type may be appropriately decided depending on the design. For example, a top gate transistor having a semiconductor layer formed from monocrystalline silicon and a bottom gate transistor having a semiconductor layer formed from amorphous silicon may be formed on the same substrate so as to provide transistors with different characteristics to the same substrate (for example, see Patent Literature 1).
Currently, from the viewpoint of reducing the cost and the picture frame size, a gate monolithic circuit having a pixel part and a driving circuit part on the same panel is being developed. For example, methods are studied for producing a TFT at pixel part and a TFT at driving circuit part simultaneously.